The invention relates to a control system according to the preamble of claim 1. The invention further relates to a continuously variable transmission, alternatively denoted CVT, provided with such a control system. The control system and CVT are known, for example, from EP-A-0,787,927 and are particularly suited for application in motor vehicles.
The known CVT comprises a V-type drive belt wound around a first pulley provided on a first shaft and a second pulley provided on a second shaft. Both pulleys have two conical discs, an axial separation of which is adjustable by means of a piston/cylinder assembly of the pulleys. During operation of the transmission the drive belt is clamped with a first clamping force between the discs of the first pulley and with a second clamping force between the discs of the second pulley, whereby each clamping force is effected by pressurised hydraulic medium in the piston/cylinder-assembly of the respective pulleys. The level of the clamping forces determines the maximum torque level that can be transmitted between the first and the second shaft virtually without relative movement in the tangential direction between the drive belt and the discs of the pulleys occurring. Such relative movement is alternatively denoted as belt slip and is highly undesirable because it may lead to considerable wear of the drive belt. The said maximum torque level is referred to as the torque transmitting capacity of the CVT. In the known CVT a speed ratio of the rotational speeds of the first and second shafts is inversely related to the ratio of the running radii of the drive belt between the discs of the first and the second pulley, which may be changed by changing the ratio of the clamping forces.
The CVT is provided with a control system for determining a first cylinder pressure in the piston/cylinder assembly of the first pulley and a second cylinder pressure in the piston/cylinder-assembly of the second pulley. The known control system comprises a pump for generating a flow of hydraulic medium from a reservoir for hydraulic medium to the said piston/cylinder assemblies and at least two valves that are positioned in parallel, i.e. the flow generated by the pump essentially passes through either one of the said two valves. The control system is arranged such that the first cylinder pressure, the so-called line pressure, is controlled to a desired level by means of a first, or line pressure control valve and that the second cylinder pressure is set by means of a second valve. In the known control system the second valve is a flow valve that controls the said second cylinder pressure by allowing either a smaller or larger flow of medium from the first hydraulic line, where the said line pressure prevails, a smaller or larger flow of hydraulic medium from the second cylinder to the reservoir, where the prevailing pressure is relatively low, e.g. atmospheric pressure, or no flow of hydraulic medium at all. Consequently, the second cylinder pressure may in principle be set at a value anywhere in the range between the pressure in the reservoir and the line pressure.
Although the control system of the known CVT is capable of achieving adequate clamping force levels for realising a desired torque transmitting capacity and speed ratio of the CVT, it was found in practice that the accuracy and the stability with which the known control system controls the second cylinder pressure is not optimal. Moreover, particularly when the CVT is at standstill, i.e. when the shafts are not rotating, it proves to be difficult or even impossible to estimate and/or calculate the second cylinder pressure with a satisfying accuracy and without actually measuring it, which latter option is considered undesirable and expensive. In these circumstances, a sufficiently high second cylinder pressure can not be guaranteed by the known control system and the second cylinder pressure can unwittingly become so low that belt slip may occurs, either at standstill or immediately thereafter when the vehicle, in which the CVT is applied, is accelerating. This may disadvantageously affect the functioning of the CVT and, in particular, may reduce the service life of the drive belt.
It is an object of the present invention to improve the accuracy and stability with which the second cylinder pressure may be controlled and more in particular it is an object to reduce the risk of belt slip of the drive belt at standstill and/or during initial acceleration of a motor vehicle in which the CVT is applied.
According to the invention this object bay be achieved by a control system having the features discussed below. In the control system according to the invention the second valve or controlling the second cylinder pressure is influenced not only by a control pressure, but also by the second cylinder pressure itself, e.g. by creating a pressure feedback. Such influence may improve the accuracy of the second cylinder pressure control. Moreover, belt slip at or immediately after standstill may be effectively prevented with the control system according to the invention.
An insight underlying the invention is that in the known control system the actual level of the second cylinder pressure is determined and influenced indirectly by the amount of flow to and from the second cylinder. The said amount of flow may be very small in cases where there only exists the requirement to compensate for pressure losses due to leakage, i.e. steady state conditions, and in particular at stand still. The known second valve, i.e. the flow valve that is typically operated by a control pressure was found to be not particularly suite for accurate steering of such a small amount of flow. However, by allowing feedback of the second cylinder pressure, i.e. by involving the level of the second cylinder pressure on its own control, there may be realised a pressure control of the second cylinder pressure without the requirement of far-reaching modifications to the control system. It is a merit of the present invention that it drastically improves the functioning of the control system at standstill through a relatively easily implemented measure. A particularly advantageous control system may be realised by a negative influence of the feedback of the second cylinder pressure, i.e. by realising the said influence such that changes in the second cylinder pressure are counteracted. In this manner a stable level of the second cylinder pressure is achieved, independent of temporary fluctuations therein.
In a further development of the invention, the second valve for controlling the second cylinder pressure is additionally influenced by the line pressure. In case of the continuously variable transmission this is to be preferred, because usually when either one of the second cylinder pressure and the line pressure change the other pressure is desired to change too. According to the invention, it is in this case particularly advantageous if the influence of the line pressure is such that an increase or a decrease in the line pressure is reflected by an increase or a decrease in the second cylinder pressure respectively. The proportion between the line pressure and the second cylinder pressure may accordingly influence the second cylinder pressure. It was found that within the context of the present invention with the latter configuration the best results in terms of pressure stability and of pressure control accuracy may be achieved.